CTSL and viral infectious disease: There are the structural viral Spike (S1) protein, which plays a critical role in the initial steps of pathogenesis, the viral entry receptors, such as the human angiotensin‐converting enzyme 2 (ACE‐2) receptor, tyrosine‐protein kinase receptor UFO (AXL) kinase, and host cell receptor CD147, which mediate viral infection by binding to the Spike (S) protein, the serine protease Transmembrane Serine Protease 2 (TMPRSS2), Furin and Cathepsin L (CatL), for Spike protein priming and facilitating the fusion of the viral and host membranes.